I want to refer to the timeframe of Summer 2019 in the Northern hemisphere. However, the writing is intended for a global audience, and when it is Summer in the Northern hemisphere it will be Winter in the southern hemisphere.
Is there terminology I can use that refers to that period of time in both hemispheres?
The common phrase is
While readers in the northern hemisphere may understand this to more narrowly mean summer in the northern parts of whichever continent they live on – as in the title of Sir John Carr's book A Northern Summer; Or, Travels Round the Baltic, Through Denmark, Sweden, Russia, Prussia, and Part of Germany, in the Year 1804 –, readers in the southern hemisphere will think of this in the way you intend, and certainly none of them will think of Africa or Australia.
In the natural sciences, "northern summer" does have exactly the meaning you seek, as can be seen in this quote from Jaffe & Taylor's The Physics of Energy from 2018:
And when the precession index e sin 𝜛 is at a maximum, so that Northern summer coincides with perihelion while eccentricity is at a maximum, the Northern Hemisphere also receives more summer insolation.
There are countless examples for this use in the sciences, but it is maybe more interesting to note that the term is used by Australian institutions when referring to what you might call "global seasons", as in this Australian government website on international aviation (my emphasis):
The timetable summary provides information on airlines approved to operate scheduled services between Australia and its bilateral partners for the Northern Winter period from 28 October 2018 to 30 March 2019.
In technical writing it is important to be precise. If you want to state something, it should generally be stated in the clearest language possible. In a comment you say:
I very specifically need to refer to the timeframe of June-July-August.
and since you want a replacement for "Summer 2019" I suggest one of the following:
Why not state a season?
You say you want to be specific, but using a season in place of the specific months is not very specific. If you state the months, readers know exactly what you mean. If you say "summer" it is open to interpretation. This is true even if your audience is local. Does summer mean the date range starting the day after spring ends, and ending the day before fall begins? If so, whose definition of those dates should be used? It isn't the same everywhere. Or maybe it means the period of time when it feels like summer, which may or may not match up reasonably well with the specific date range? It isn't clear.
Why not say "northern summer"?
This, in my opinion, is even worse. It has the same specificity problems of stating a season, but with additional confusion. First, it relies on the reader knowing when summer is in a different part of the world. Maybe they can figure it out, but its an unnecessary obstacle for them to understand your document.
On top of that, without reading the other answer, I wouldn't know what "northern summer" means. I live in the northern United States. If I had to guess, I would have guessed it means summer in Canada, which maybe is supposed to mean a shortened summer? It is unnecessarily confusing, and doesn't even convey the information to a fully informed reader.
To further emphasize this point, here are some alternate interpretations that I don't think are too much of a stretch:
Even if the reader eventually settles on the correct interpretation, too much time was spent trying to determine what was meant when a simple June-August would have been much clearer.
When can I use a season?
I don't think stating a time period as a season is always to be avoided. It can be used when a characteristic of the season is what is important. If you were writing documentation about a solar panel, which will provide more power during the long daylight days of the summer, it would be perfectly reasonable to say "The highest power output can be expected during the summer". In this case what is important isn't the particular date range, but instead a characteristic of the season. This makes using the season name more appropriate. I can't at the moment think of any reason I would want to use "northern summer", but if I did, I would instead phrase it as "summer in the northern hemisphere", since, as previously mentioned, I find "northern summer" to be unclear.
1. Maybe someone can help me better express this. I'm referring to what happens when a term that looks like adjective-noun does not modify the noun, but instead refers to a different noun.
Many global corporations just say Q1, Q2, Q3. So Q3 2019.
There are a few simple ways to do that.
‘The second season of the year’. It feels a tad uncomfortable but it works.
You can simply say which month or date it is. Regardless of where you are in the world the dates stay the same* (so December is warm in the southern hemisphere while July is cold).
‘Halfway through the year’ would be summer in the north and winter in the south.
*other than the international date line, but that is just a 1 day difference so it doesn’t really make a difference.
However, the Northern summer suggestion is probably better. Upvote that answer instead.
A lot of writers seem to assume that the "four seasons" are universal. They may occur at different times but everyone has them. In reality a substantial proportion of the world's population live in the tropics, where temperatures and hours of daylight are fairly uniform throughout the year. There may be "wet" or "dry" or "monsoon" seasons but no "summer" or "winter". Some places have almost no seasonal variation.
I suspect that not many of your anticipated readership live in tropical rain forests, but I am an not sure that this justifies ignoring them completely.
In any case what is understood by a particular season depends on the person and the context. For some people each season begins at a definite date, for others it depends on the weather. Could you not refer to the season, the date and the weather in a natural way that would still be understood by someone with different seasons. Perhaps something like: "Winter came early that year. The first snow fell before the end of October and most people had dug out their warm clothing at least a month earlier.".
You should follow whatever is the convention in the field you are writing for.
Does it matter that it's summer (such as when writing about meteorology, climatology, ecology) or is it purely a time indicator?
For example, in climate science, the conventional phrase is "northern hemisphere (NH) summer" or "northern hemisphere winter" in text. In a figure, the same would often be abbreviated with "JJA" or "DJF", which in a figure legend or table heading would be understood by this audience, but probably not by a different audience. Either use would mean the northern hemisphere summer (or winter) over multiple years, such as in "northern hemisphere summer temperatures 1960-2010". For the northern hemisphere summer in a particular year, one might write "northern hemisphere summer 2018" but more likely just "June-August 2018". I've never seen "northern summer" in this context, and I do think it would be confused to not cover, say, Mexico.
In a different context, I've also seen quarters used. Such as: the test data will be available in Q1 2019. The satellite will be launched in Q3 2022. This processor was introduced Q3 2015. Again, you should only use this if your intended audience is familiar with this notation and if it is unambiguous.
In my field we say "boreal summer." Here is a source: http://thingsnotfoundonwikipedia.blogspot.com/2014/09/boreal-spring-boreal-summer-boreal.html
